Perceiving dissonances is partly and largely a neurological
and physical thing. When two frequencies that form a dissonant interval
are heard simultaneously the ear's critical bands overlap. However, when
someone sings or creates music in their mind, i.e. creating and hearing
music internally and not externally, one can still hear dissonance.

This brings a question to my mind. Do we actually physically
hear what we think? When we hear sounds in our head are our ears actually
physically working? Are our critical bands activated?

I remember a statement made by a friend that scientists
found that some schizophrenics actually showed physical ear activity when
they "heard voices". This would back up the answer to my previously
stated question:

Because dissonance occurs when our critical bands overlap
then does it also occur when we think or hear the dissonance in our mind?